The use of microporous membranes as battery separators is known. For example, microporous membranes are used as battery separators in lithium-ion batteries. Such separators may be single layered or multi-layered thin films made of polyolefins. These separators often have a ‘shut-down’ property such that when the temperature of the battery reaches a predetermined temperature, the pores of the membrane close and thereby prevent the flow of ions between the electrodes of the battery. Increasing temperature in the battery may be caused by internal shorting, i.e., physical contact of the anode and cathode. The physical contact may be caused by, for example, physical damage to the battery, damage to the separator during battery manufacture, dendrite growth, excessive charging, and the like. As such, the separator, a thin (e.g., typically about 8-25 microns thickness) microporous membrane, must have good dimensional stability.
Dimensional stability, as it applies to battery separators, refers to the ability of the separator not to shrink or not to excessively shrink as a result of exposure to elevated temperatures. This shrinkage is observed in the X and Y axes of the planar film. This term has not, to date, referred to the Z-direction dimensional stability.
Puncture strength, as it applies to battery separators, is the film's ability to resist puncture in the Z-direction. Puncture strength is measured by observing the force necessary to pierce a membrane with a moving needle of known physical dimensions.
To date, nothing has been done to improve the Z-direction dimensional stability of these battery separators. Z-direction refers to the thickness of the separator. A battery is tightly wound to maximize its energy density. Tightly winding means, for a cylindrically wound battery, that forces are directed radially inward, causing a compressive force on the separator across its thickness dimension. In the increasing temperature situation, as the material of the separator starts to flow and blind the pores, the electrodes of the battery may move toward one another. As they move closer to one another, the risk of physical contact increases. The contact of the electrodes must be avoided.
Accordingly, there is a need for a battery separator, particularly a battery separator for a lithium-ion battery, having improved Z-direction stability.
In the prior art, it is known to mix filler into a separator for a lithium battery. In U.S. Pat. No. 4,650,730, a multi-layered battery separator is disclosed. The first layer, the ‘shut down’ layer, is an unfilled microporous membrane. The second layer, the dimensionally stable layer, is a particulate filled microporous layer. The second layer, in final form (i.e., after extraction of the plasticizer), has a composition weight ratio of 7-35/50-93/0-15 for polymer/filler/plasticizer. There is no mention of Z-direction dimensional stability; instead, dimensional stability refers to the length and breadth dimensions of the separator. The filler is used as a processing aid so that the high molecular weight polymer can be efficiently extruded into a film. In U.S. Pat. No. 6,432,586, a multi-layered battery separator for a high-energy lithium battery is disclosed. The separator has a first microporous membrane and a second nonporous ceramic composite layer. The ceramic composite layer consists of a matrix material and inorganic particles. The matrix material may be selected from the group of polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyurethane, polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polytetraethylene glycol diacrylate, copolymers thereof and mixtures thereof. The inorganic particles may be selected from the group of silicon dioxide (SiO2), aluminum oxide (Al2O3), calcium carbonate (CaCO3), titanium dioxide (TiO2), SiS2, SiPO4, and the like. The particulate makes up about 5-80% by weight of the ceramic composite layer, but most preferably 40-60%. There is no mention of Z-direction stability, and the particulate is chosen for its conductive properties.